What structure in the region of the lungs where the majority of gas exchange occurs?
a. terminal
b. bronchiole alveolus
c. alveolar duct
d. respiratory
e. bronchiole
f. bronchus

b. bronchiole alveolus

Explanation:

The lungs is the major organ for respiration. The lungs contain cells and tissues which aid gaseous exchange. The alveolus(plural:alveoli) is the major structure in which gaseous exchange takes place.

The option lacks alveoli and it is understood that very little gaseous exchange takes place in the alveolar duct then we choose bronchiole alveolus since the alveoli are formed by branching out from the bronchiole.